1. Field of the Invention
The present invention relates to a mode locking semiconductor laser apparatus employed in long distance, large capacity optical fiber communication or the like to generate an ultra short optical pulse train.
2. Description of the Related Art
The prior art technologies in this field include those disclosed in the following publications:
Reference 1; ELECTRONICS LETTERS 31 (10) (May 11, 1995)
A. Asahira and Y. Ogawa; xe2x80x9cPassive and Hybrid Mode Locking in a Multi-Electrode DBR Laser with Two Gain Sectionsxe2x80x9d page 808xcx9c809
Reference 2; Transcripts of the 59th Applied Physics Academy Lectures, Fall term V.1.0, (1998)
Yoshida and Nakazawa; Ultrashort pulse generation with a high repetition rate from a soliton fiber laserxe2x80x9d
Methods of generating ultra short optical pulse trains in the prior art include the mode locking method. In the mode locking method, an optical pulse train is generated by lasing light in multi-longitudinal modes and locking the phases of the individual longitudinal modes. To achieve mode locking by locking the phases of the longitudinal modes, it is necessary to assume a structure of a resonator and to modulate the gain or the loss in the resonator with an orbital frequency determined in conformance to the length of the resonator or a frequency that is achieved by multiplying the orbital frequency by an integer (hereafter, these frequencies are referred to as mode locking frequencies).
The hybrid mode locking method achieved by applying a modulation voltage with a frequency that roughly matches a mode locking frequency to a saturable absorber is a type of mode locking method. By adopting the hybrid mode locking method, the pulse width suppression effect achieved by the saturable absorber and the advantage of time jitter reduction achieved by applying the modulation voltage are realized at the same time so that an ultra short optical pulse train with little time jitter can be generated. In reference 1 above, a semiconductor laser constituted by integrating a saturable absorber, a gain area and a diffraction grating is employed and a reverse bias voltage is modulated by using the orbital frequency (8.68 GHz) and then is applied to the saturable absorber, to successfully generate an ultra short optical pulse train with a pulse width at 8.3 ps and a low time jitter of 0.2 ps.
A mode locking frequency, which is determined by the optical length of the resonator constituting the laser, may fluctuate due to a change in the optical length of the resonator resulting from disturbances such as heat from the outside. Such a fluctuation in the mode locking frequency is not desirable since it may cause an error in signal identification on the reception side in an optical communication system, for instance. In reference 2, a fiber laser type regenerating mode locking laser is disclosed. In this regenerating mode locking laser, a frequency deviation relative to the frequency generated by a reference frequency generator such as a synthesizer is detected by adopting a PLL (phase locked loop). Then, by performing fine adjustment on the fiber loop length, which is equivalent to the resonator length, so as to correct the frequency deviation, a stable fiber laser type mode locking laser that sustains a constant mode locking frequency over long period of time is provided.
As described above, a semiconductor laser which promises to be ideal in application as the light source in optical communication and optical information processing not only needs to generate an ultra short optical pulse train with a low time jitter but also is required to withstand a thermal disturbance and the like and to output a stable, ultra short optical pulse train over a long period of time.
Accordingly, in the mode locking semiconductor laser that generates an ultra short optical pulse train having a frequency corresponding to a reference frequency in a first embodiment of the present invention, an optical resonator is constituted by forming an active wave guiding channel to which a modulation signal at the reference frequency is applied, a passive wave guiding channel that is transparent to oscillated light and an ohmic resistance heating film is provided over the passive wave guiding channel that generates Joule heat based upon an injected current and changes the refractive index of the passive wave guiding channel to change the optical length, and the following structure is assumed.
Namely, it is provided with a photoelectric transducer that converts the ultra short optical pulse train to an electrical signal and a phase comparator that compares the phases of the electrical signal and the modulation signal and detects the deviation of the frequency of the ultra short optical pulse train relative to the reference frequency based upon their phase difference, and the current injected into the ohmic resistance heating film is changed so as to eliminate the frequency deviation detected by the phase comparator.
By adopting this structure, the mode locking semiconductor laser operates while achieving hybrid mode locking and generates an ultra short optical pulse train with a frequency corresponding to the reference frequency. If the optical length of the resonator changes due to a disturbance caused by heat or the like, the phase comparator detects a frequency deviation in the frequency of the ultra short optical pulse train relative to the reference frequency in correspondence to the phase difference between the electrical signal output by the photoelectric transducer and the modulation signal and the current injected into the ohmic resistance heating film is controlled so as to eliminate this frequency deviation. In other words, the optical length is adjusted.
The ohmic resistance heating film may be constituted of Pt (platinum), Ti (titanium), an alloy of Pt and Ti or the like. The injected current may be varied by using a variable resister or by changing a source voltage that generates the injected current. In addition, a filter that allows the frequency component corresponding to the mode locking frequency of the electrical signal to pass may be provided between the photoelectric transducer and the phase comparator.
In the mode locking semiconductor laser that generates an ultra short optical pulse train having a frequency corresponding to a reference frequency in a second embodiment of the present invention, an optical resonator is constituted by forming an active wave guiding channel to which a modulation signal at the reference frequency is applied, a passive wave guiding channel, which is transparent to oscillated light and ohmic resistance heating film provided over the passive wave guiding channel that generates Joule heat based upon an injected current and changes the refractive index of the passive wave guiding channel to change the optical length, and the following structure is assumed.
Namely, it is provided with a phase comparator that compares the phases of the photoelectric current flowing through the active wave guiding channel and the modulation signal applied to the active wave guiding channel and detects the frequency deviation of the photoelectric current and the modulation signal based upon their phase difference, and the current injected into the ohmic resistance heating film is changed so as to eliminate the frequency deviation detected by the phase comparator.
By adopting this structure, operation is performed while achieving hybrid mode locking and an ultra short optical pulse train with a frequency corresponding to the reference frequency is generated. If the optical length of the resonator changes due to a disturbance caused by heat or the like, the phase comparator detects a frequency deviation in the frequency of the ultra short optical pulse train relative to the reference frequency in correspondence to the phase difference between the photoelectric current flowing through the active wave guiding channel and the modulation signal, and the current injected into the ohmic resistance heating film is controlled so as to eliminate this frequency deviation. In other words, the optical length is adjusted.
The ohmic resistance heating film may be constituted of Pt, Ti, an alloy of Pt and Ti or the like. The injected current may be varied by using a variable resistor or by changing a source voltage that generates the injected current. In addition, an amplifier that amplifies the photoelectric current to flow through the active wave guiding channel may be provided between the active wave guiding channel and the phase comparator.
In the mode locking semiconductor laser that generates an ultra short optical pulse train having a frequency corresponding to a reference frequency in a third embodiment of the present invention, an optical resonator is constituted by forming an active wave guiding channel to which a modulation signal at the reference frequency is applied and a passive wave guiding channel, which is transparent to oscillated light and by applying a control voltage to the passive wave guiding channel that changes the refractive index in order to change the optical length, and the following structure is assumed.
Namely, it is provided with a photoelectric transducer that converts the ultra short optical pulse train to an electrical signal and a phase comparator that compares the phases of the electrical signal and the modulation signal and detects the deviation of the frequency of the ultra short optical pulse train relative to the reference frequency based upon their phase difference, and the control voltage is changed so as to eliminate the frequency deviation detected by the phase comparator.
By adopting this structure, the mode locking semiconductor laser operates while achieving hybrid mode locking and generates an ultra short optical pulse train with a frequency corresponding to the reference frequency. If the optical length of the resonator changes due to a disturbance caused by heat or the like, the phase comparator detects a frequency deviation in the frequency of the ultra short optical pulse train relative to the reference frequency in correspondence to the phase difference between the electrical signal output by the photoelectric transducer and the modulation signal and the control voltage is changed so as to eliminate this frequency deviation. In other words, the optical length is adjusted.
It is to be noted that a filter that allows the frequency component corresponding to the mode locking frequency of the electrical signal to pass may be provided between the photoelectric transducer and the phase comparator.
In the mode locking semiconductor laser that generates an ultra short optical pulse train having a frequency corresponding to a reference frequency in a fourth embodiment of the present invention, an optical resonator is constituted by forming an active wave guiding channel to which a modulation signal at the reference frequency is applied and a passive wave guiding channel, which is transparent to oscillated light and applying a control voltage to the passive wave guiding channel that changes the refractive index to change the optical length, and the following structure is assumed.
Namely, it is provided with a phase comparator that compares the phases of the photoelectric current flowing through the active wave guiding channel and the modulation signal applied to the active wave guiding channel and detects the frequency deviation of the photoelectric current and the modulation signal based upon their phase difference, and the control voltage is changed so as to eliminate the frequency deviation detected by the phase comparator.
By adopting this structure, operation is performed while achieving hybrid mode locking and an ultra short optical pulse train with a frequency corresponding to the reference frequency is generated. If the optical length of the resonator changes due to a disturbance caused by heat or the like, the phase comparator detects a frequency deviation of the frequency of the ultra short optical pulse train relative to the reference frequency in correspondence to the phase difference between the photoelectric current flowing through the active wave guiding channel and the modulation signal, and the control voltage is controlled so as to eliminate this frequency deviation. In other words, the optical length is adjusted.
It is to be noted that an amplifier that amplifies the photoelectric current to flow through the active wave guiding channel may be provided between the active wave guiding channel and the phase comparator.